1. Field of the Invention
This invention relates to apparatus for measuring properties of energetic materials. More specifically, the present invention relates to ceramic liners for use in closed bomb testing of energetic materials such as propellant, explosive, and pyrotechnic formulations.
2. Technology Background
It is often desirable to evaluate the effects of raw materials and processing variables on the functioning of energetic materials. In most cases, it is impractical to manufacture full scale items and test them under field conditions. Closed bomb testing was developed in the early 1960""s to provide a low cost, rapid, and effective alternative to full scale testing.
Closed bomb testing was initially used in the development of process controls for rocket propellant manufacturing. The intent was to determine ballistic characteristics of the propellant before it was cast into the motor case. Closed bomb testing is now routinely used for other energetic materials such as pyrotechnic and explosive compositions.
A typical closed bomb device has been prepared by modifying a Parr oxygen bomb calorimeter. The inlet and outlet are modified to allow prepressurization of the bomb with an inert gas and to permit attachment of a strain gage transducer for measurement of pressure-time data. Means for igniting the energetic material is also provided. To reduce thermal loss, the interior of the bomb is lined with silicone rubber. A primer is used on the metal bomb body and the silicone rubber is applied to the bomb interior and cured. The bomb head is also lined with the rubber.
It will be appreciated that any vessel with the required pressure rating can be adapted to closed bomb testing by providing an inlet/outlet for pressurization, a pressure measuring device such as a transducer, and a means for igniting the sample energetic material.
In practice, the bomb is pressurized with an inert gas, such as argon. The energetic material is ignited and the pressure change over time is measured. The burn rate is then calculated from the pressure data by dividing the sample thickness by the apparent burn time (the time from ignition peak to maximum pressure). When the burn rate is plotted against the pre-load pressure on a logarithmic scale, the slope equals the burn rate exponent. Impetus, the work a propellant can do in ft-lb/lb, may also be determined from the pressure data, and the characteristic velocity of a propellant can be related to its impetus.
Impetus values are useful in determining the available energy in a propellant or pyrotechnic. When the impetus is known, it is possible to calculate the maximum pressure in a given volume from the combustion of a known weight of sample. Using the impetus value, the actual hazard of an accidental ignition can be reasonably assessed. For example, when an energetic composition is ignited, the only danger may be from a slow thermal energy release with little danger of over-pressurization and explosion. In other cases, relatively small amounts of composition may produce a rapid pressure build-up that will result in a violent explosion.
To obtain accurate pressure data, it is highly desirable to operate a closed bomb under adiabatic conditions (no heat loss to surroundings). As mentioned above, silicone rubber has been used as an internal liner in an effort to obtain adiabatic conditions.
Silicone rubber, however, has several significant disadvantages. For example, silicone rubber is compressible. Because the test results are based upon a constant volume system in which the actual volume is known, compression of the liner alters the volume and introduces error into the test results. To compensate for such errors, various assumptions and empirical corrections are made to the resulting data in an attempt to make the results look normal.
Other disadvantages of silicone rubber liners are the necessity to cast the liner and the difficulty of bonding the liner to the bomb walls. To apply the silicone rubber liner, the bomb is spun on a lathe and a uniform layer of the silicone rubber is manually placed on the interior surface of the bomb. The bomb is then spun at high rpm under a heat lamp for several hours to cure the rubber. To help the silicone adhere to the bomb interior, a primer coating is often applied. Nevertheless, the silicone liner often peels off of the bomb interior after a few shots. Because a large number of shots are required to obtain data on a single energetic material, the gradual deterioration of the silicone liner over time makes it difficult to obtain reproducible data.
Another major problem associated with silicone rubber liners is the poor durability of the liner, necessitating frequent replacement. The silicone liners degrade in the energetic environment within the closed bomb. After several shots, the liners need to be replaced. Finally, current silicone rubber liners allow significant amounts of heat loss which introduces errors into calculations and makes it more difficult to correlate the measured data.
Accordingly, it would be an advancement in that art to provide closed bomb devices for measuring properties of energetic materials having liners which have low compressibility, low thermal conductivity, and high durability.
Closed bomb devices for measuring performance properties of energetic materials having such liners are disclosed and claimed herein.
The invention is directed to the use of a ceramic liner in closed bomb devices for measuring performance properties of energetic materials. Alumina silicate and boron nitride are two currently preferred ceramic materials which have low compressibility and low thermal conductivity. These ceramic materials may be machined such that the liner can be accurately and conveniently prepared to fit within the closed bomb device. It has been found that these ceramic materials have excellent durability. The term energetic materials, as used herein, includes solid propellant, explosive, and pyrotechnic formulations.